Musnad Ahmad 25:908

Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Amr ibn al-Harith informed me, from Bukayr ibn al-Ashajj, from Kurayb, from Ibn Abbas: That he saw Abdullah ibn al-Harith praying while his hair was braided behind his head. So he stood behind him and began to undo it, and the other [man] submitted to him. Then he [Abdullah] turned to Ibn Abbas and said: "What is the matter with you and my head?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The likeness of this is only like the likeness of one who prays while he is bound.'"